To raise pH in a hot tub, add a small dose of soda ash (spa “pH Up” or “pH Increaser”) with the jets running and the cover off, then test again after 20 minutes. Hot tubs are small: about 20 grams (under an ounce) takes a 400-gallon tub from pH 7.2 to 7.6 when alkalinity is 80 ppm. Aim for 7.4–7.8, and raise alkalinity first if it is under 50 ppm.
How much pH Up your hot tub needs
Pool
Hot tub
Hot tub size
gal
pH now
How acidic the water is. Right, chlorine works and eyes do not sting; wrong, it damages the pool.
pH you want
Good range: 7.4–7.8.
Alkalinity (if you know it)
ppm
It decides how far a dose moves pH. Empty = a typical 80.
Stabilizer (if you know it)
ppm
Stabilizer buffers pH too. Empty = a typical 30.
Using pH up.
Type the size and your pH reading.
Step by step
1
Test pH and alkalinity. The good range in a hot tub is pH 7.4–7.8.
2
If alkalinity is under 50 ppm, raise it first with baking soda.
3
Work out the dose with the calculator on this page.
4
With the jets running and the cover off, sprinkle it in front of a jet.
5
Test again after about 20 minutes, and add more only if pH is still low.
The product label always wins — if it says something different, follow the label.
Why it happens
Hot tub pH falls with the acid you added to bring it down, some sanitizers, and low alkalinity that lets it swing.
Questions
Why does my hot tub pH keep dropping?
Most often low alkalinity. Keep alkalinity at 50–80 ppm and pH holds steady.
Can I use baking soda to raise hot tub pH?
Baking soda raises alkalinity much more than pH. Use it when alkalinity is low; use soda ash when only pH is low.
Can I use pool pH Up in a hot tub?
Yes — it is the same soda ash. Just use the much smaller hot tub amount.